C# 使用单例模式打开新的winform

我有这样一个表单，我想使用singleton来确保只有一个实例在运行

private static productForm instance;
public productForm()
{
    InitializeComponent();
}

public static productForm GetInstance
{
    get
    {
        if (instance == null)
        {
            instance = new productForm();
        }
        return instance;
    }
}

/*private void productForm_FormClosed (object sender, FormClosingEventArgs e)
{
    instance = null;
}*/

}

在mainform中单击事件

    private void categoryTreeToolStripMenuItem_Click(object sender, EventArgs e)
    {
        productForm.GetInstance.Show();

    }

关闭productform时出现错误，尝试重新打开时出现错误

productForm.GetInstance.Show（）的名称；将输出错误消息“无法访问已处置对象”，似乎productForm不可访问，有人知道问题出在哪里吗

试试这个

private static productForm instance;
public productForm()
{
  InitializeComponent();
}

public static productForm GetInstance
{
get
{
    if (instance == null || instance.IsDisposed)
    {
        instance = new productForm();
    }
    return instance;
}
}

试试这个

private static productForm instance;
public productForm()
{
  InitializeComponent();
}

public static productForm GetInstance
{
get
{
    if (instance == null || instance.IsDisposed)
    {
        instance = new productForm();
    }
    return instance;
}
}